For eons of time insects have been in natural competition with plants. Insects depend on plants for food and consequently threaten the plants' survival. Plants develop new chemical insecticides to protect themselves from the insects. The insects, in turn, develop new methods to metabolize the plant insecticides.
The ability of insects populations to rapidly adapt to pesticides is also apparent today, as many insects of agricultural and medical importance have become resistant to control with man-made insecticides. No class of insecticide chemistry has been exempt from this process, and, as insecticides have become widely used, a variety of insecticide-selected insect genes with unique properties toward chemical metabolism have developed.
In contrast to insects, soil-inhabiting microorganisms have not been exposed to this type of environmental pressure, and generally have not developed mechanisms of degrading insecticides like that of insects. Thus, many insecticides (e.g., Aldrin, Chlordane, DDT, HCH) are notoriously persistent in soil (See, e.g., R. Stanier et al., The Microbial World, 557-58 (5th Ed. 1986)). Such persistent insecticides are referred to as "recalcitrant insecticides," and have half lives in soil which are measured in periods of years, rather than periods of days. See M. Alexander, Introduction to Soil Microbiology, pg. 445 (2d Ed. 1977).
Insecticides are critically important to modern agriculture. Their value is offset, however, by the tendency of soil to become contaminated for prolonged periods of time with insecticides which are no longer effective in controlling insects. There is a continued need for new solutions to this problem.